The polarization vector has the clear physical meaning in the rest frame of the particle, therefore the precession of vector \(\vec{\xi }\)  is usually considered.
In the instantaneous rest frame the polarization vector obeys the classical equation [1]}
\(\frac{d \vec{\xi }}{d \tau } = - \frac{e g}{2m } \vec{H}^\star \times \vec{\xi },\) 
